Decades of research reveal new understanding of how climate change may affect Arctic soil carbon stocks

Using one of the longest-running ecosystem experiments in the Arctic, a team of researchers led by Colorado State University has developed a better understanding of the interactions between plants, microbes and soil nutrients—results that provide new insights into how carbon deposits may be important. released from melting Arctic permafrost.

Estimates show that Arctic soil contains almost twice the amount of carbon that is currently in the atmosphere. As climate change has caused parts of the northern hemisphere to melt, scientists have long been concerned about large amounts of carbon being released through greenhouse gases, a process fueled by microbes.

Much of the effort to study and model this scenario has focused primarily on how global warming will disrupt the carbon currently locked in Arctic soils. But warming is affecting the region in other ways, too, including changing plant productivity, the overall composition of vegetation in the landscape, and the balance of nutrients in the soil. These changes in plant composition will also affect how carbon is transported from the soil to the atmosphere, according to research published this week in the journal. Climate Change.

The work was led by Megan Machmuller, a research scientist in CSU’s Department of Soil and Crop Sciences.

“Our work aimed to identify the mechanisms that are responsible for controlling the fate of carbon in the Arctic,” Machmuller said. “We know temperature plays a big role, but there are also ecosystem changes that are happening in conjunction with climate change in this region.”

In particular, Machmuller said, the area is experiencing a type of “shrub saturation” – an increase in the amount of bushes and growth. And what Machmuller and his co-authors found is that in the long run those bushes can contribute to keeping more carbon in the soil.

“There’s been a lot of focus on the direct effects of warming on soil carbon,” co-author Laurel Lynch, an assistant professor at the University of Idaho, “but what we’re finding with this work is that it’s much more complex. The need to think about this ecosystem as a whole community with many interacting parts and competing systems.”

Amazing discovery

For the new work, Machmuller and team tested soil samples from a 35-year ecosystem experiment in the Arctic. In 1981, scientists began adding nutrients to test plots at the Long-Term Arctic Ecological Research site in northern Alaska, located near Toolik Lake at the base of the Brooks Mountain Range. The original idea was to understand how Arctic vegetation would respond to additional nutrients over time, but the experiment has also allowed scientists to examine how long-term changes in soil can affect carbon storage.

After 20 years, scientists found that there was a significant loss of soil carbon when nutrients were added compared to control plots, an important finding that built a broader scientific understanding of how the Arctic can adapt to climate change. The experiments continued, and Machmuller and his team tested the plots again after 35 years of continuous nutrient use.

Instead of continuing to lose carbon, however, they found that the situation had changed. After 35 years, the amount of carbon stored in the experimental plots had matched or exceeded the amount in nearby control plots.

“We were very surprised by these results and were curious about the underlying mechanism,” Machmuller said.

Machmuller and his team conducted advanced isotope-tracing experiments in the laboratory to learn more about how carbon was moving through the system. What they found was that when the nutrients were first added, they stimulated microbial decomposition—a natural process that involves microbes breaking down organic matter in the soil and resulting in the release of carbon dioxide.

But that changed over time, as nutrients were constantly added to the test plots. “Shrubs conditioned the soil in a way that changed microbial metabolism, reduced decomposition rates and allowed the soil’s carbon stock to rebuild,” Lynch said. “We didn’t expect that.”

“This provides a possible biological mechanism that could explain why we saw carbon loss in the first 20 years but not after 35,” Machmuller said.

The importance of looking long term

These results, Machmuller said, show that how the Arctic may respond to climate change is more complex than previously thought. “It’s a complex puzzle,” he said, “and this study has emphasized to us the importance of using long-term studies to advance our understanding of ecosystem processes.”

Gus Shaver, a research scientist who helped establish the Toolik Lake experimental sites in 1981 and is a co-author of the study, also emphasized the value of doing this kind of work over a longer period of time.

“We’ve shown that long-term experiments produce frequent surprises when we track their responses over time,” Shaver said. “What you get in the first few years of an experiment is often not what you learn from year 10 or 15 or 35.”

Lynch noted that as this ecosystem changes, there are other factors to consider than just carbon. While increased shrub growth can prevent more soil carbon from being transferred to the atmosphere, other effects are not as beneficial, he said.

“When you have one plant species that is overwhelmingly overpowering the entire community, there are huge ecosystem impacts,” Lynch said. For example, he said, “habitats and food sources for many animals in the Arctic depend on diverse plant communities, and this type of loss can spread throughout the entire ecosystem.”

Lauren Gifford, associate director of CSU’s Soil Solutions Center, who was not involved in the research, said the work highlights the need for more robust and detailed modeling to better anticipate how climate change will affect carbon stored in the Arctic.

“This is an incredible 35-year study of one of the world’s most endangered ecosystems,” Gifford said. “Even with comprehensive long-term studies, the effects of climate change often remain uncertain. Measures to help adapt to climate change can lead to results that are consistent, contradictory, or produce unexpected results.”

For his part, Machmuller hopes the work will encourage future research on this topic. “Carbon research in the Arctic has long been a hot topic because of the important role it plays in controlling our global climate,” he said. “But we still don’t know what the future carbon balance will be.”